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Transcript
Status of the LIGO Experiment
Keith Riles
University of Michigan
(representing the LIGO Scientific Collaboration)
Aspen Winter 2003 Conference on Particle Physics:
At the Frontiers of Particle Physics
Aspen Center for Physics – January 19-25, 2003
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Nature of Gravitational Waves

Gravitational Waves = “Ripples in space-time”

Perturbation propagation similar to light
 Velocity = c
 Two transverse polarizations - quadrupolar:

+ and x
Amplitude parameterized by (tiny) dimensionless strain h
DL ~ h(t) x L
Example:
Ring of test masses
responding to wave
propagating along z
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Why look for Gravitational Radiation?

Because it’s there! (presumably)

Test General Relativity:
 Quadrupolar radiation? Travels at speed of light?
 Unique, quantitative probe of strong-field gravity

Gain different view of Universe:
 Sources cannot be obscured by dust
 Detectable sources some of the most interesting,
least understood in the Universe
 Opens up entirely new non-electromagnetic spectrum
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What will the sky look like?
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Generation of Gravitational Waves

Radiation generated by quadrupolar mass movements:
(with Imn = quadrupole tensor, r = source distance)

Example: Pair of 1.4 Msolar neutron stars in circular orbit of radius 20 km
(imminent coalescence) at orbital frequency 400 Hz gives 800 Hz
radiation of amplitude:
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Generation of Gravitational Waves
Major expected sources in 10-1000 Hz band:

Coalescences of binary compact star systems
(NS-NS, NS-BH, BH-BH)

Supernovae
(requires asymmetry in explosion)

Spinning neutron stars, e.g., pulsars
(requires axial asymmetry or wobbling spin axis)
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Generation of Gravitational Waves

Sources well below LIGO bandwidth:





Binaries well before coalescence
Inspiral of stars into massive black holes
Coalescence of massive black holes
Stochastic background (e.g, big bang remnant, superposition of binaries)
Irreducible seismic noise argues for space-based system
at low frequencies - LISA
Three satellites forming
interferometers with 5 x 106 km
baselines (launch after 2010?)
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Generation of Gravitational Waves

Strong indirect evidence for GW generation:
Taylor-Hulse Pulsar System (PSR1913+16)
Two neutron stars (one=pulsar)
in elliptical 8-hour orbit
Measured perihelion advance
quadratic in time in agreement with
absolute GR prediction
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Generation of Gravitational Waves
Can we detect this radiation directly?
NO - freq too low
Must wait ~300 My for
characteristic “chirp”:
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Generation of Gravitational Waves
Coalescence rate estimates based on two methods:
 Use known NS/NS binaries in our galaxy (two!)
 A priori calculation from stellar and binary system evolution

Large uncertainties!
For initial LIGO design “seeing distance” (~20 Mpc):
Expect 1/(3000 y) to 1/(4 y)
 Will need Advanced LIGO to ensure detection
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Gravitational Wave Detection
Two search methods used to date – Bars and interferometers
 Suspended Resonant Bars: (pioneered by J. Weber)
 Narrow band (f0 ~ 900 Hz, Df ~ 1-10 Hz – present detectors)
 Look for sudden change in amplitude of thermally driven resonance
 No wave form information
Allegro
detector at
LSU
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Gravitational Wave Detection

Suspended Interferometers (IFO’s)
 Broad-band (~50 Hz to few kHz)
 Waveform information
(e.g., chirp reconstruction)
 Michelson IFO is
“natural” GW detector
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Gravitational Wave Detection
Major Interferometers coming on line world-wide
LIGO (NSF-$300M)
Livingston, Louisiana &
Hanford, Washington
2 x 4000-m
1 x 2000-m
Commissioning /
Data Taking
VIRGO
Near Pisa, Italy
1 x 3000-m
In construction /
Commissioning
GEO
Near Hannover, Germany 1 x 600-m
TAMA
Tokyo, Japan
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1 x 300-m
Commissioning /
Data Taking
Commissioning /
Data Taking
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Gravitational Wave Detection
LIGO Optical Layout
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Gravitational Wave Detection
Initial LIGO Design Sensitivity
Dominant noise sources:
•Seismic below 50 Hz
•Suspensions in 50-150 Hz
•Shot noise above 150 Hz
Best design sensitivity:
~3 x 10-23 Hz-1/2 @ 150 Hz
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LIGO Scientific Collaboration (~400 scientists)
Caltech
LIGO Hanford Observatory
LIGO Laboratory
MIT
LIGO Livingston Observatory
LIGO Livingston LIGOLA
University of Adelaide ACIGA
LIGO Hanford LIGOWA
Australian National University ACIGA
Loyola New Orleans
Balearic Islands University
Louisiana State University
California State Dominquez Hills
Louisiana Tech University
Caltech LIGO
MIT LIGO
Caltech Experimental Gravitation CEGG
Max Planck (Garching) GEO
Caltech Theory CART
Max Planck (Potsdam) GEO
University of Cardiff GEO
University of Michigan
Carleton College
Moscow State University
Cornell University
NAOJ - TAMA
Fermi National Laboratory
Northwestern University
University of Florida @ Gainesville
University of Oregon
Glasgow University GEO
Pennsylvania State University
NASA-Goddard Spaceflight Center
Southeastern Louisiana University
University of Hannover GEO
Southern University
Hobart – Williams University
Stanford University
India-IUCAA
Syracuse University
IAP Nizhny Novgorod
University of [email protected]
IUCCA India
Washington State [email protected] Pullman
Iowa State University
University of Western Australia ACIGA
Joint Institute of Laboratory Astrophysics
University of [email protected]
Salish Kootenai College
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LIGO Observatories
Hanford
Observation of nearly
simultaneous signals 3000 km
apart rules out terrestrial artifacts
Livingston
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LIGO Detector Facilities
•Stainless-steel tubes
(1.24 m diameter, ~10-8 torr)
•Gate valves for optics isolation
•Protected by concrete enclosure
Vacuum System
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LIGO Detector Facilities
LASER


Infrared (1064 nm, 10-W) Nd-YAG laser from Lightwave (now commercial product!)
Elaborate intensity & frequency stabilization system, including feedback from
main inteferometer
Optics



Fused silica (high-Q, low-absorption, 1 nm surface rms, 25-cm diameter)
Suspended by single steel wire
Actuation of alignment / position via magnets & coils
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LIGO Detector Facilities
Seismic Isolation


Multi-stage (mass & springs) optical table support gives 106 suppression
Pendulum suspension gives additional 1 / f 2 suppression above ~1 Hz
102
100
10-2
10-6
10-4
Horizontal
10-6
10-8
Vertical
10-10
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Some startup troubles at Hanford…
Brush fire sweeps over site
– June 2000
Tacoma earthquake –
Feb 2001
•Misaligned optics
•Actuation magnets dislodged
•Commissioning delay
Charred landscape,
but no IFO damage!
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Human error too!
22
And at Livingston…
First access road a bit damp –
now paved and higher
Gators & schoolchildren tours don’t mix...
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And at Livingston…
A Truly Serious Problem - LOGGING
Livingston Observatory
located in pine forest popular
with pulp wood cutters
Spiky noise (e.g. falling trees) in
1-3 Hz band creates dynamic
range problem for arm cavity
control
Temporary workaround:
Boost actuation gain at cost in
attainable sensitivity
Long-term Solution:
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Retrofit with active feed-forward isolation system
(using technology developed for Advanced LIGO)
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And at Livingston…
Until actuation boosted, was nearly impossible to lock IFO on weekdays
Correlation
between seismic
noise and lock
livetime
(4-day
weekend)
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Engineering & Science Runs
A series of engineering runs (“E1” to “E8”) have been completed in order to
prepare for science runs: (E9 starts tomorrow!)
• Study detector sensitivity and calibration stability
• Shake down control room software and operational procedures
• Investigate artifacts, identify problems to be fixed
• Data have proven “rich” in artifacts (a good thing we looked!)
Science runs defined by priority given to maximizing performance &
reliability (as opposed to “trying things”):
• “S1” science run in August/September 2002 (17 days)
• “S2” science run starts February 14, 2003 (59 days)
• “S3” science run starts late 2003 (~6 months)
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Astrophysical Data Analysis
• Four “upper limits groups” organized for E7 run (as practice)
(now working on S1 data, getting ready for S2)
•Inspiraling binary systems
•Unmodelled bursts
•Periodic sources (e.g. pulsars)
•Stochastic background (e.g., big-bang remnant)
•Present status:
• Preliminary limits on sources under review by collaboration;
• Results to be presented soon and submitted for publication
• No surprises (astrophysical ones, anyway…)
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E7 Data Analysis
Sensitivity curves for the three interferometers: (Jan 2002)
Many orders of
magnitude to go!
“Straightforward”
to reduce
Much harder! (instrumental artifacts, many potential noise sources)
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E7 Data Analysis
Viewing injected inspiral “chirp” via spectogram and “Rayleigh monitor”
(top plots for GW channel, bottom for an auxiliary laser channel)
Chirp easy to see (good)
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Instrumental artifacts
easy to see too! (bad)
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E7 Data Analysis
Histograms of band-limited RMS in
several bands of interest
Outliers indicate non-Gaussianity,
primarily broad-band transients
 “Glitches”
Lots of work to reduce these…
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S1 Strain Sensitivities (red = simulation)
10
10
10
10
10
10
10
10
10
-13
-14
-15
-16
-17
-18
-19
-20
-21
10
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LHO 4k
LHO 2k
LLO 4k
SimLIGO
1
10
2
10
3
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4
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S1 Data Analysis
Despite these
improvements, occasional
artifacts appeared:
Hanford 4-km “heartbeat”
Tracked down to
undamped “side motion”
of one end mirror kicked
up by Oregon earthquake
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S1 Data Analysis
(three hours of Livingston 4-km)
Histograms vs time of
filtered GW channel
SLIGHTLY RAGGED DATA
CLEAN DATA
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S1 Data Analysis – “Inspiral Range” (kpc)
Calibration not as stable as we want: (one day of S1 Hanford 4-km running)
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S1 Data Analysis
Non-stationarity, glitches, etc., make analysis delicate
But one can estimate very rough expected upper limit
sensitivities from preliminary work:
Stochastic backgrounds
–Upper limit 0 < ~30 (energy density!)
Neutron binary inspiral
–Upper limit distance < ~200 kpc
Known pulsar
–Upper limit h < ~10-21
Results for presentation / publication in early 2003
S2 should give substantial improvement in h
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Looking back one year
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Looking Ahead
The three LIGO interferometers will be part of a global network.
Multiple signal detections will increase detection confidence and
provide better precision on source locations and wave polarizations
LIGO
GEO
Virgo
TAMA
AIGO
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Looking Ahead
TAMA (300-meter – Japan):
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Looking Ahead
Virgo (3000-meter – Italy/France):
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Looking Way Ahead
Despite their immense technical challenges, the initial LIGO IFO’s
were designed conservatively, based on “tabletop” prototypes, but
with expected sensitivity gain of ~1000.
Given the expected low rate of detectable GW events, it was always
planned that in engineering, building and commissioning initial LIGO,
one would learn how reliably to build Advanced LIGO with another
factor of ~10 improved sensitivity.
Because LIGO measures GW
amplitude, an increase in
sensitivity by 10 gives an
increase in sampling volume,
i.e, rate by ~1000
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Advanced LIGO
Detector Improvements
Increased laser power: 10 W  180 W
Improved shot noise (high freq)
Increased test mass: 10 kg  30 kg
Compensates increased radiation pressure noise
New test mass material: Fused silica  Sapphire
Lower internal thermal noise in bandwidth
New suspensions: Single  Quadruple pendulum
Lower suspensions thermal noise in bandwidth
Improved seismic isolation: Passive  Active
Lowers seismic “wall” to ~10 Hz
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Advanced LIGO
Sampling of source
strengths vis a vis Initial
LIGO and Advanced LIGO
Lower hrms and wider
bandwidth both important
“Signal recycling” offers
potential for tuning shape
of noise curve to improve
sensitivity in target band
(e.g., known pulsar range)
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Advanced LIGO
Ambitious upgrade program:
•MRE proposal for NSF now in preparation
•Hope to begin detector upgrades in 2006
 Begin observing in 2007
First 2-3 hours of Advanced LIGO is equivalent
to a Snowmass year of Initial LIGO
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Summary
Initial LIGO commissioning well underway
Much instrumental noise to beat down, but no show-stoppers
have appeared
Confronting realities of dirty-data analysis
Engineering runs giving way to sporadic science runs (with
astrophysical measurements as primary purpose)
interspersed with ongoing commissioning
Looking ahead to several years of high-duty-cycle data taking
Looking farther ahead to major detector upgrade with more than
1000-fold increase in event rate
Direct GW detection only a matter of time
Exciting years to come!
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